Key fobs, or remote keyless entry devices, that unlock, for example, the driver's door, passenger doors, or the trunk lid are well known. Some key fobs control other user-preferred features such as seat position, radio station, and air control temperature settings. Many key fobs are manually activated by the user (active approach) as the vehicle is approached. Other key fobs transmit a signal in response to a low frequency query from the vehicle (passive approach), with the key fob signal being detected by the vehicle for activating the desired features.
Low frequency passive systems require continuous (periodic) transmission and typically have a limited range of less than two meters. However, the low frequency transmission communication period must be sufficiently long to reduce the current consumption of the transmission, while being short enough to allow a noticeable approach to activate the feature before the user arrives at the vehicle. These low frequency systems may provide less time than desired for the activation of, for example, approach lighting. As a result the user is already at or very near the vehicle when approach lighting is activated.
Furthermore, low frequency passive systems can cause unintentional actuations when the user is near the vehicle but does not desire to activate the functions. These unintentional activations cause an undesired drain on the vehicle and fob batteries and may create a security issue if the unintentional actuation renders the vehicle accessible. Low frequency passive systems may include provisions to deactivate the approach sensing after extended continuous activation; however, this may have the undesired result of not providing the vehicle user the expected operation when desired.
The use of vehicle based sensors or trigger criteria solve these unintentional actuations. However, these require the user to initiate features at the vehicle, which initiates the fob. Furthermore, advanced activation of approach lighting is typically unavailable.
Accordingly, it is desirable to provide a system that enables passive approach detection using a unidirectional fob and reduces parasitic current consumption of a vehicle while managing fob battery life.